(1) Field of the Invention
The present invention relates to an organic electrolyte cell and, more particularly, to an improvement in an organic electrolyte cell having bismuth trioxide, Bi.sub.2 O.sub.3, used as the positive electrode active material.
In organic electrolyte cells having lithium as the negative electrode active material, various cells differing in the operating voltage can be fabricated, but from the practical viewpoint, these cells are roughly divided into a 3-V type having an operating voltage of about 3 V and a 1.5 V type having an operating voltage of about 1.5 V. As the positive electrode active material for the 1.5-V type, there are known cupric oxide, iron sulfide, iron disulfide, lead oxide and bismuth trioxide. A cell fabricated by using bismuth trioxide has a high operating voltage of 1.5 to 1.8 V and this cell is especially advantageous for the use where a large electric current discharge is needed. Moreover, the operating voltage of an organic electrolyte cell comprised of lithium as the negative electrode active material and Bi.sub.2 O.sub.3 as the positive electrode active material is about 1.5 V since this cell is substantially equivalent in the operating voltage to a conventional ordinary commercial cell using an aqueous alkali electrolyte, such as an alkali-manganese cell or a silver oxide cell, the organic electrolyte cell has an interchangeability with such a conventional cell. Furthermore, since the creep phenomenon which would be observed in the alkali electrolyte is not caused in the organic electrolyte, leakage of the electrolyte is reduced, so that the self-discharge is small and the energy density is high. Because of these advantages, development of a cell having an excellent long-period reliability and a high energy density is expected.
(2) Description of the Related Art
In the case where a cell of this type, for example, a button cell is prepared, the positive electrode is formed in the following manner. Namely, bismuth trioxide as the active material, an electrically conductive material, for example, a carbon powder such as graphite or carbon black or a metal powder and a resin binder such as a fluororesin or polystyrene are mixed together at a predetermined ratio, and a predetermined amount of this positive electrode mixture is packed in a mold of a molding machine and compression-molded to obtain a pellet-shaped positive electrode molded body. The so-obtained positive electrode pellet is heated and dried under reduced pressure at a temperature (up to about 300.degree. C.) not causing thermal decomposition of the resin binder or oxidation of the conductive agent and, after the positive electrode is thus dehydrated sufficiently, the positive electrode is assembled into a cell. Organic electrolyte cells containing bismuth trioxide as the positive electrode active material are disclosed, for example, in Japanese Patent Application Laid-Open specification No. 12425/1977 and Japanese Patent Publication No. 49673/1984 which corresponds to U.S. Pat. No. 4,085,259.
As the organic electrolyte of the cell of this type, there have been used solutions of ionizable salts such as LiClO.sub.4, LiBF.sub.4, LiPF.sub.6 and LiSO.sub.3 CF.sub.3 as the supporting electrolyte in aprotic organic solvents such as propylene carbonate (hereinafter referred to as "PC"), .gamma.-butyrolactone, tetrahydrofuran, 1,2-dimethoxyethane (hereinafter referred to as "DME"), dioxolane and mixtures thereof.
In general, the electrolyte of the cell should have various characteristics and in view of the cell performance the following properties are most important.
(1) The ion conductivity is high and the electrode reaction speed is high. PA1 (2) The boiling point is high, the solidification point is low, and the range of the operating temperatures is broad. PA1 (3) The electrolyte is stable against the positive electrode and negative electrode materials and the like, and the solubilities of the positive electrode and negative electrode materials are low. PA1 (4) The decomposition voltage is high.
An organic electrolyte formed by dissolving LiClO.sub.4 in a mixed solvent of PC having a high boiling point, a low solidification point, a high dielectric constant and a high solubility of the supporting electrolyte and of DME having a low viscosity is excellent in the foregoing properties and therefore this organic electrolyte has been frequently used.
As the electrically conductive material for the positive electrode of the cell of this type, there are known carbon powders such as graphite and acetylene black (carbon black which is called "cell black", prepared by thermal decomposition of acetylene) and metal powders.
When the discharge is carried out in a cell fabricated according to the above-mentioned conventional process by using bismuth trioxide as the positive electrode material, two discharge voltages are observed and a lower operating voltage is obtained at a discharge depth larger than 40%. Accordingly, when this cell is used for an instrument in which a large electric current discharge is needed, a voltage necessary for normally operating the instrument cannot be obtained because of the lowered cell voltage even though a sufficient cell capacity is left.
Especially in case of such an instrument as an electronic watch or an electronic desk-top computer, an average consumption current is as small as several .mu.A and the life required for a cell is as long as several months to 10 years or longer. Also, a pulsating large current discharge is necessary for operating a lamp, a buzzer, a step motor and the like. A cell to be used for this purpose is required to have a long-period reliability and a large current discharge capacity. Therefore, the abovementioned problem is very serious.